uniform sampler2DRect tex;
void main(void){
   vec2 uv = gl_TexCoord[0].st;
   float f = length(texture2DRect(tex,uv).rgb);
   if (f > 0.5) f = 1.0;
   else f = 0.0;
   gl_FragColor = vec4(vec3(f), 1.0);
}
